Hey Paulears, yeah thanks for the advice... you mention it not being a typical hum.... well, doing some research I'm beginning to find some news about the effects of 4G signals... we've not long had 4G nationally introduced here in The Netherlands - pretty much the last few months.... and this is where I hope I'm not getting ahead of myself; but an LCD TV on the first floor has been getting strange lines in it's picture over the last few weeks - as it's a number of years old, my initial reaction was, oh it's on it's way out, they're not built to last forever... but, strange lines and TV interference has been positively attributed to 4G signals from local antennas - according to some sources... now I hope I'm not putting 2 and 2 together and getting 22, but as my problem seems to have quite an unusual sound (as you mention not the typical hum), and it's seemingly consistent regardless of the hardware config I use, I'm really wondering about this... it will take some further research, but if there's anybody out there who maybe has heard of anything to do with 4G interference just let me know.

Don't forget that we know quite a bit about what devices are problematic with 4G, because many of the frequencies were in use by TV channels in the UK and Holland for many years - here, we lost our radio microphone allocation in channel 69, when it was re-used by the 4G companies. Pretty well the only issue is the other way around - interference TO people who have illegally carried on using channel 69 - suddenly a quiet band is full of nasty, loud digital signals. Your crackles in the noise floor (because they are pretty low) don;t have any of the characteristics of digital transmissions. I run quite a few higher powered radio systems, and now these are a mix of analog and digital. I have a repeater system here that runs pretty reasonable power, and the only detectable interference is a freeze in the digital (and cheap) TV receiver in my edit suite. The audio studio is closer to the aerial, and suffers no issues whatsoever.

TV interference is certainly possible from 4G - there is one area in my home town that is suffering badly from a 4G installation that manages to wipe out TV pictures. It's been mainly cured by a simple filter - but the visible problem is simply no picture - the old analog system had all sorts of visible patterning until it totally broke up. Digital has a much narrower range between perfect, and gone!

I suspect the problem is MUCH simpler. Have you tried a sound source direct into the monitoring system? You must be systematic and not random. Step by step. Something in your chain of equipment is generating the noise - my guess is that it will be the interface with the computer. Plugging an iPod direct into the computers inputs will hopefully be noise free. Then connect the same iPod (because it runs on batteries) to the interface. Press the buttons and see if the noise is there. If it is - it's being generated either in the interface, or by a faulty driver, although that's less likely.

As I said before - if you do this, and there is no noise, then start to add a microphone - perhaps a cheap dynamic if you have one to use without phantom power. If this is noise free, add your better condenser, apply power and see.

If it helps, you can occasionally get this kind of noise from a quality condenser when the phantom power supplied is not sufficient. The phantom power system is always termed 48V, but some interfaces can't manage quite that, or perhaps can, but only with a low current demand from the mic. Some mics, while within the phantom specification, draw quite a lot, and if it's not available, the voltage falls, and the preamp inside the mic cannot perform properly, and occasionally manifest this with the type of noise you have experienced.

So, we're a bit stuck until you have the time to do some serious and time consuming testing.